The present invention relates to an extrusion molding system and method.
When manufacturing a continuous extrusion with a sectional shape varying continuously along the length direction, such as automotive window molding, the variable section of the extrusion can be obtained by partly cutting an extruded work with a predetermined sectional shape by a cutter, the cutter being successively moved by an actuator to change a cutting position of the work.
With this method, since the rotation speed of a screw of an extruder is maintained constant, the discharge amount of resin can be controlled easily, providing moldings with high accuracy in a stable way. However, chips are produced in quantity, leading to an increase in material cost.
One solution is proposed in JP-A 6-71730 wherein an extrusion molding system comprises a shape changing plate arranged at an opening of a die to change the shape thereof, an actuator connected to the plate for reciprocation thereof, and an extruder having screw rotation speed sequence-controlled in accordance with change in sectional shape of the opening due to movement of the plate so as to change the discharge amount of resin.
Another solution is proposed in JP-B2 6-55415 wherein an extrusion molding system comprises a gear pump arranged between a die and an extruder, a pressure sensor arranged prior to the gear pump, and extruder having screw rotation speed feedback-controlled so that the pressure indicated by the pressure sensor becomes equal to a set pressure.
However, with the solution of JP-A 6-71730, the change in discharge amount of resin does not adequately follow change in sectional shape of the die opening. And even with the screw rotation speed of the extruder varied, the discharge amount of resin is not changed linearly, resulting in difficult forming of moldings with larger rate of change. Moreover, even when the extruder rotates at constant speed, the accuracy of the discharge amount of resin varies ±3%. And when changing the rotation speed continuously, the ±3% accuracy is further lowered, resulting in no satisfaction of a desired value of accuracy.
With the solution of JP-B2 6-55415, it is gear pump and not extruder which finally feeds resin to the die, wherein the gear pump has a discharge amount of resin which is merely followed by the change in amount of resin fed from the extruder with low accuracy in terms of time and amount, and is not controlled along the time axis. With the method of conducting feedback control by detecting pressure variations, no problem occurs when the discharge amount of resin has a relatively small rate of change, such as at startup of the system and change in set conditions. However, when manufacturing automotive moldings with sectional shape varied within the range of a mere few hundred millimeters, an electronic control unit (ECU) cannot provide quick and sufficient calculations, causing possible occurrence of breakdown of the gear pump due to pressure increase or poor filling of resin due to pressure decrease, resulting in difficult achievement of a required rate of change of the discharge amount.